The big question is how committed to this are you? And what do you consider a reasonable budget?
Realistically you can get a decent airbrush for $75 or less. I would suggest starting witha dual action airbrush. They give you more control over your paint and make it easier to do fades and shadows and to apply different thicknesses of paint. There are a lot of popular ones. The Paasche VL set is one that a lot of us use. That's what I learned on. I just replaced it with an Iwata Revolution HP-C and I like it. I know Badger also makes a nice dual action set but I can't remember the model # off the top of my head. Honestly, the compressor is the most pricey part of all this...
Hit the search for "airbrush" and you should find plenty of threads debating this.